PROJECT SUMMARY/ABSTRACT Preterm infant brain injury is often associated with blood-brain barrier (BBB) disruption and altered maturation of the central nervous system (CNS) progenitor cells required for normal brain development and myelination. The molecular signals in the injured microenvironment that inhibit CNS progenitor maturation are not fully known. Thus, no therapeutic options are available to prevent the developmental disabilities associated with preterm birth. BBB disruption alters the CNS progenitor niche by allowing blood proteins into the CNS. Fibrinogen, a blood coagulation protein, crosses a leaky BBB and is a key contributor to neuroinflammation, glial scar formation, neurodegeneration, and inhibition of CNS repair. We hypothesize that fibrinogen is a critical component of the microenvironment in preterm infant brain injury that inhibits CNS progenitor cell maturation to impair brain growth and myelination. Our preliminary studies show: (1) neonatal mice subjected to chronic hypoxia display prominent cerebellar pathology that includes fibrinogen deposition, myelination deficits, and impaired cerebellar growth; (2) Intraventricular injection of fibrinogen in neonatal mice disrupts cerebellar development in vivo; (3) Fibrinogen activates the bone morphogenetic protein (BMP) receptor activin A receptor type I (ACVR1) in oligodendrocyte progenitor cells (OPCs) to inhibit OPC maturation and myelination, (4) Fibrinogen inhibits neurogenesis from neuronal progenitor cells in vitro. Our specific aims will test our working model, whereby fibrinogen deposition after BBB disruption induces ACVR1-mediated BMP signaling in CNS progenitor cells to inhibit neurogenesis and myelination leading to abnormal neurodevelopment. In Aim 1, we will define the contribution of fibrinogen to preterm infant brain injury in vivo using fibrinogen mutant mice. In Aim 2, we will identify the cellular mechanisms that contribute to preterm infant brain injury at sites of fibrinogen deposition using in vivo two-photon microscopy (2PM). In Aim 3, we will determine the molecular mechanism of fibrinogen-induced activation of ACVR1 using in vitro binding and cellular assays. These studies will reveal the molecular link between BBB disruption and failure of CNS progenitor cell maturation in preterm infant brain injury. My goal is to become an independent physician- scientist and leader in the field of newborn brain injury. To continue my progress towards this goal, I will build upon the expertise of the Gladstone Institutes and UCSF to expand my research skills in the following areas: (1) in vivo 2PM to study the dynamic cellular responses to BBB disruption and fibrinogen deposition in the neonatal brain, (2) RNA-sequencing and transcriptome analysis to identify fibrinogen-mediated mechanisms of extrinsic inhibition, and (3) binding assays and inhibitor studies to discover novel fibrinogen receptors on CNS progenitor cells. The knowledge and experience gained from this proposal will allow me to compete for R01 funding focused on therapeutics that target the inhibitory microenvironment in preterm infant brain injury.